The altitude does affect things, since the pressure gauge on your regulator reads in psi gauge, or psi relative to the ambient pressure.
At sea level, the ambient air pressure is 14.7 psi (one atmosphere), so if I set my regulator to 20 psig I'll be applying 34.7 psi of absolute CO2 pressure. At 5280 feet, your ambient air pressure is around 12 psi (the general rule is you subtract 0.5 psi for every 1000 feet of elevation gain). So if you set your regulator to 20 psi, you'll be applying 32 psi of absolute CO2 pressure. Got it?
Basically, you want to choose a carbonation level and use a carbonation chart or calculator (I like this one) to figure out the pressure based on your beer's temperature. Then, add about 2.5 psi to that number. Make sure you add a little bit to your line length to help balance the extra pressure. 12 feet of standard 3/16" ID vinyl beer line should be fine.
Here's another thread where someone had a similar question: